The spectrum of the sky-averaged 21 cm brightness temperature, or the so-called global 21 cm signal,
provides valuable information on the early history of structure formation, from the dark ages to the cosmic reionization
(e.g. [1]}).
The EDGES experiment has reported the detection of an absorption trough with a depth of
\(\delta T_{\rm 21} = -500^{+200}_{-500}\, {\rm mK}\)  (99% confidence level) corresponding to the cosmic dawn
[2]}, which is unexpectedly large as compared with theoretical predictions from the standard model.
Although the claimed signal may be affected by instrumental effects (e.g. [3]}),
mis-modeled foregrounds (e.g. [4]}), or data analysis systematics (e.g. [5]}),
various theoretical works have been trying to explain the signal level by introducing new physics.
These models invoke a variety of mechanisms to explain the large absorption, e.g. extra-cooling of the cosmic gas (e.g.
[6]}; [7]}; [8]};
[9]}; [10]}; [11]};
[12]}; [13]}; [14]}), extra-source of early radio
background in addition to the cosmic microwave background (CMB) (e.g. [15]}, [16]}, [17]}), or modified Hubble expansion rate [18]}, [19]}.
